1. Field of the Invention
The present invention relates to a collision detector which detects a collision of an automobile.
2. Description of the Related Art
In the prior art, a collision detector to detect a collision of an automobile is known (disclosed in Japanese Unexamined Patent Application Publication No. Hei 3-20674, for example).
Recently, an automobile is equipped with an occupant protector unit such as an airbag system in order to secure safety of occupants at a collision.
The airbag system is configured to explode an airbag in receiving a high impact over a predetermined amount of force and use it for a cushion for the head of an occupant so as to protect the occupant from the impact.
FIG. 1 shows a prior art collision detector 1 which is placed in an airbag sensor unit below a vehicle dashboard and comprises sensor units SU1, SU2, a comparator 2, a determiner circuit 3, and an activator circuit 4.
Also, an airbag module 5 is provided in the center of a steering wheel, for example, and mainly comprised of an airbag and an inflator to explode the airbag.
The sensor unit SU1 is comprised of an acceleration sensor S1 and an amplifier A1 while the sensor unit SU2 is comprised of an acceleration sensor S2 and an amplifier A2. The acceleration sensors S1, S2 detect acceleration of the vehicle at their respective positions to output a signal with a voltage which is in proportion to a detected acceleration.
The acceleration sensor S1 is placed over a front side member to detect a front collision of the vehicle while the acceleration sensor S2 is placed in the airbag sensor unit for the purpose of improving reliability of collision detection.
The acceleration sensors S1, S2 are disposed to detect the acceleration in the same direction relative to a front-back direction of the vehicle. Output terminals of the acceleration sensors S1, S2 are connected with input terminals of the amplifiers A1, A2 to have inverse polarities so that in receiving acceleration in the front-back direction of the vehicle, the sensor units SU1, SU2 generate inverse output potentials relative to GND.
The comparator 2 includes an input terminal 2a for positive feedback, an input terminal 2b for negative feedback and an output terminal 2c. The input terminals 2a, 2b are connected with the output terminals of the amplifiers A1, A2 respectively, and the output terminal 2c is connected with the determiner circuit 3.
The comparator 2 amplifies a signal with a potential difference V+− between a potential of V+ of the input terminal 2a and that V− of the input terminal 2b and output it to the output terminal 2c. 
The determiner circuit 3 determines whether or not the vehicle is colliding according to a predetermined potential difference V+− between a potential of V+ of the input terminal 2a and that V− of the input terminal 2b, that is, between the output potential V+ of the sensor unit SU1 and that V− of the sensor unit SU2.
Upon determining that a signal from the output terminal 2c of the comparator 2 is due to a collision, the determiner circuit 3 outputs a signal to the activator circuit 4. The activator circuit 4 activates the airbag module 5 to instantaneously explode the airbag, receiving the signal.
Meanwhile, various electric noises constantly occur in the automobile from wiring of an ignition system or the like, for example.
The prior art collision detector 1 has a problem that these electric noises may enter signal paths between the acceleration sensors S1, S2 and the comparator 2, and cause erroneous activation of the airbag module 5.
The airbag system originally aims to protect occupants from a vehicle collision, however, the airbag system may be erroneously activated during vehicle running.
In the prior art collision detector 1, when the acceleration sensors S1, S2 receive the same acceleration, the output potentials V+ (FIG. 2A), V− (FIG. 2B) of the sensor units SU1, SU2 become inverse to each other relative to GND, for example.
The potential difference V+− between the potentials V+, V− is a value obtained by adding absolute values |V+|, |V−| of the potentials V+, V− (FIG. 2C). The collision detector 1 determines that the vehicle is colliding when the potential difference V+− is a predetermined voltage threshold Vth or more.
Electric noises in the vehicle are mostly electromagnetic noises, and noise signals entering the signal paths between the acceleration sensors S1, S2 and the comparator 2 often have a similar signal pattern.
When the signal paths receive the electric noises, the output potential V+ of the sensor unit SU1 (FIG. 3A) and the output potential V− of the sensor unit SU2 (FIG. 3B) vary in a similar pattern relative to GND over time.
In this case, a difference between the potentials V+, V− will be a difference between their respective absolute values |V−|, |V+|, which is a very small value (FIG. 3C). These electric signals due to noises are generally negated with each other so that the collision detector 1 is prevented from activating the airbag module 5 in response to the electric signals.
However, as shown in FIGS. 4A, 4B, in a case where noise signals entering the signal path between the acceleration sensor S1 and the comparator 2 and those entering the signal path between the acceleration sensor S2 and the comparator 2 have different signal patterns, these noise signals cannot be negated with each other. This may cause a problem of erroneously activating the airbag module 5 since the potential difference V+− is a large value as shown in FIG. 4C.